Theme 1: Lecture 4 - Mobility of the GI tract Flashcards
What is the role of the GI tract
to extract chemical energy, vitamins, minerals and water from ingested products
What are the basic 4 layers of the GI tract
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
What is in the mucosa layer of the GI tract
- Epithelium
- Lamina propria
- Muscularis mucosae
What is in the submucosal layer of the GI tract
- Submucosal nerve plexus
- Blood vessels and lymphatic supply
What is in the muscularis externa layer of the GI tract
- Circular muscle
- Myenteric nerve plexus
- Longitudinal muscle
Describe how the motility of the GI tract is controlled
- Motility is governed by involuntary contraction of smooth muscle with pacemaker interstitial cells of Cajal (ICC)
- Except upper oesophagus and external anal sphincter (striated skeletal muscle/voluntary)
Describe the smooth muscle in the GI tract
- Smooth muscle is single unit- gap junctions allow electrical coupling and contraction as a functional syncytium
- Smooth muscle organised into connected bundles of outer longitudinal and inner circular smooth muscle in muscularis layer
What is the intrinsic enteric nervous system
- It controls GI motility and secretion independently of external neurostimulation
- Reflex contraction in response to local stimuli (stretch, nutrients, irritation, hormones)
What are the 2 interconnected plexuses in the gut wall
- Myenteric plexus
- Submucosal plexus
What is the extrinsic nervous control of the GI tract
- Autonomic sympathetic and parasympathetic innervation
- Allow central modification to the GI tract
Describe the myenteric plexus
- AKA Auerbach’s plexus
- In the muscularis layer
- Controls motility
Describe the submucosal plexus
- AKA Meissner’s plexus
- In the submucosal layer
- Controls secretion and local blood flow
Describe the parasympathetic innervation of the GI tract
excitatory to motility and secretion (via Vagus and pelvic splanchnic nerves)
Describe the sympathetic innervation of the GI tract
Inhibitory to motility and secretion (via thoraco-lumbar innervation)
Describe how endocrine hormones enter the portal blood circulation
Endocrine hormones are secreted by entero-endocrine cells in the epithelial layer of the GI mucosa and enter portal blood circulation
Name 2 hormones that affect GI motility
- Cholecystokinin CCK
- Motilin
Describe cholecystokinin
- The stimulus for secretion is fat, protein and acid
- It is secreted from I cells of the small intestine
- Stimulates pancreatic secretions, gallbladder contraction and growth of exocrine pancreas. Inhibits gastric emptying
Describe motilin
- The stimulus for secretion is fat, acid and nervous
- Secreted from the M cells of the duodenum and the jejunum
- Stimulates gastric and small intestine motility
What are the 2 types of electrical activity in smooth muscle cells of the gut
- Slow waves
- Spike potentials
Describe slow waves
- cyclical oscillations of membrane potential spontaneously initiated by pacemaker ICCs
- Don’t reach threshold potential so don’t cause contractions in themselves
- Provide a basic electrical rhythm
Describe spike potentials
- Generated once threshold is reached resulting in Ca2+ influx and smooth muscle contraction
- Causes contraction by further depolarisation to threshold levels
What is depolarisation of smooth muscle in the gut stimulated by
- stretch
- hormones (motilin)
- excitatory neurotransmitter acetylcholine release from ENS excitatory motor neurons or P/S
What causes inhibition of the smooth muscle in the gut resulting in hyperpolarisation
- inhibitory ENS
- sympathetic NT norepinephrine
- hormones (secretin)
What are the 2 types of contraction that occur in the gut
- Segmentation
- Peristalsis
Describe segmentation contraction in the gut
- For mixing
- Bursts of circular muscle contraction and relaxation
- Back and forth pendular movements also occur
- stretch receptors trigger myenteric stimulation of muscle contraction
- No net movement
Describe peristalsis contraction in the gut
- For propulsion
- local distension triggers contraction behind bolus and relaxation in-front
- Wave of contraction
- Requires functional myenteric plexus
- Law of the intestines: movement aborally (away from mouth)
Name a disease resulting from ENS innervation dysfunction
Hirschsprung’s disease
What is Hirschsprung’s disease
- A rare congenital absence of the myenteric plexus, usually involving a portion of the distal colon
- The pathologic aganglionic section of colon lacks peristalsis and undergoes continuous spasm, leading to functional obstruction and severe constipation
What type of epithelium is the oesophagus lined by
Stratified squamous epithelium
What are the 3 stages of swallowing (deglutition)
- Oral - voluntary initiation of swallowing in the oral cavity
- Pharyngeal – involuntary passage of food through pharynx into oesophagus
- Oesophageal – involuntary passage of food from pharynx to stomach
Describe the oral phase of swallowing
- Under voluntary control
- tongue pushes up against hard palate and contracts to force lubricated bolus into the pharynx
- Bolus enters the oropharynx initiating the pharyngeal stage through stimulation of sensory receptors
Describe the oesophageal phase of swallowing
- Motor efferents in trigeminal, glossopharyngeal and vagal nerves cause series of muscle contractions moving bolus through oropharynx into laryngopharynx and into oesophagus (nasopharynx is not involved in swallowing)
- Soft palette elevates over posterior nares to close nasal pharynx
- Epiglottis closes larynx
- Respiration is inhibited
- Upper oesophageal sphincter relaxes
- Pharyngeal muscle contraction propels bolus into oesophagus
Where is the swallowing centre
In the medulla oblongata and the pons in the brain stem
Describe the oesophageal phase of swallowing
- Primary peristalsis moves bolus downwards
- Circular muscle contracts behind bolus, longitudinal muscle contracts in front to shorten fibres and push wall outward
- Mucus lubricates and reduces friction
- Relaxation of the lower oesophagus and lower oesophageal sphincter (LOS) occurs
- Secondary peristalsis stimulated by stretch
- Coordination is via intrinsic myenteric and extrinsic vagal innervation
Name 2 diseases caused by oesophageal motility dysfunction
- Achalasia
- Gastro-oesophageal reflux
What is achalasia
- LOS (lower oesophageal sphincter) fails to relax causing food to remain in oesophagus
- Cause may be vagal or myenteric defect
- Distention, inflammation, Infection and ulceration
What is gastro-oesophageal reflux
- LOS tone lost leading to flow of acidic gastric contents into oesophagus
- Inflammation, ulceration
- May be linked to hiatus hernia where portion of stomach protrudes through diaphragm into thorax causing gastric reflux
What are the 3 primary motor functions of the stomach
- Storage
- Mixing
- Emptying contents into the duodenum at a controlled rate
Rugae folds
coiled sections of tissue that exist in the mucosal and submucosal layers of the stomach. They provide elasticity by allowing the stomach to expand when a bolus enters it
Describe the storage function of the stomach
- The vagovagal reflex mediates receptive relaxation reducing muscle tone and allowing reservoir function
- The fundus (hollow part of the organ) functions primarily as a reservoir for storage of stomach contents
Describe mixing in the stomach
- Slow peristaltic waves are initiated in the body of the stomach moving stomach contents towards pyloric antrum.
- Food is forced back for further mixing and digestion. This process of propulsion and retropulsion occurs in cycles to produce chyme
Describe emptying of the stomach contents
- Highly regulated with primary inhibitory feedback signals from small intestine
- More powerful peristaltic contractions build to force chyme into the duodenum
What is the excitatory regulation for emptying of the stomach
ENS/ANS neuronal stimulation and hormones eg motilin
What is the inhibitory regulation for emptying of the stomach
ANS regulation, duodenal enterogastric reflexes and hormones eg CCK, secretin
Name 2 diseases causes by gastric motility dysfunction
- Dumping syndrome
- Gastroparesis
What is dumping syndrome
- Rapid emptying of gastric contents into the small intestine
- Occurs following ingestion of large meal after gastrectomy
- characterized by nausea, pallor, sweating, cramps, vertigo, and sometimes fainting within minutes
- May be caused by hypertonic duodenal contents causing rapid entrance of fluid
What is gastroparesis
- Stomach fails to empty
- prevents proper digestion
- Causes bloating and nausea
- May be caused by gastric cancer or peptic ulcers
- occasionally observed through impaired vagal stimulation to the stomach in severely diabetic patients who develop autonomic neuropathy
Describe how the small intestine has a large surface area
-circular folds (plicae circulares)
villi projections of the mucosa
-‘brush border’ microvilli on the epithelial cell apical surface
Describe the 2 types of mixing in the small intestine
- Mixing and circulation for maximum exposure to absorptive epithelium
- Propulsion of chyme aborally
Which hormones stimulate propulsive peristalsis in the small intestine (5)
gastrin, CCK, insulin, motilin, serotonin
Which hormones inhibit propulsive peristalsis in the small intestine
secretin and glucagon
Gastroenteric reflex
gastric distention activates myenteric plexus to promote SI (small intestine) peristalsis
Gastroileal relfex
gastric distention promotes peristalsis in the ileum to force chyme through ileocecal valve into caecum
Migrating motor complex (MMC)
- Series of peristaltic contractions, between meals, every 90 mins sweeps contents into colon
- Intrinsic enteric control, hormone motilin
- Absence can lead to bacterial overgrowth
Ileocecal valve
- Controls emptying of chyme into colon
- Prevents backflow
Name 3 types of disruption to peristalsis
- Peristaltic rush
- Paralytic ileus
- Vomiting
Peristaltic rush
mucosal irritation, ENS and ANS neural reflexes rapidly sweep contents of SI into colon
Paralytic ileus
loss of peristalsis following mechanical trauma
Vomiting (disruption to peristalsis)
reverse peristalsis initiated in distal small intestine to expel intestinal and gastric contents
Why is motility in the large intestine more sluggish
To allow optimal:
- Absorption of water and electrolytes (proximal)
- Formation and storage of faeces (distal)
- Commensal microbiome aids digestion, synthesises B and K vitamins
Describe the longitudinal muscle in the large intestine
Longitudinal muscle in muscularis thickened to form three bands, taniae coli, which tonically contract to form haustral bulges
Describe motility in the large intestine
-mixing contractions via haustral churning
Peristalsis:
- mass movements occur 2-3x per day (forceful peristaltic contractions force contents into sigmoid colon and rectum)
- gastro-colic and duodeno-colic reflexes: mass movements occur after meals on stretching via ANS
Defecation reflex
- Initiates defecation to expel faeces containing residues of digestion, bacteria, bile pigment, mucosal debris
- mass movements push faecal matter into the normally empty rectum
- stretch receptors are stimulated and activate the ENS and parasympathetic ANS
- Involuntary contraction of longitudinal muscle in the rectum opens the internal anal sphincter
- The constricted external anal sphincter is voluntarily relaxed to allow defecation
